In most cases the size of the driver means how much bass is put out i.e a 15 inch driver moves more air than a 12 inch and so produces more low end.
Wattage varies from every manufacturer who gives a different readings from Peak power, continuous power, RMS and others which can get very confusing.
If your adding a sub i’d go with the 2 12’s due to the fact the sub will be producing all the bass and the 12’s will deal with low mid and up when crossed over correctly with the sub.
I also don’t believe that the 12 puts out 1260 watts either, well it may but it won’t sound nice and will probably blow up lol
well a bigger sub with less watts can be just as loud if not louder as a smaller sub with more watts, depending on what your working with. the 15" will hit the lows a bit better and move more air, unless the smaller sub has some crazy excursion or something.
but in the end proper power and enclosure have the biggest impact.
Watt’s and loudness are not the same thing. You’re speakers loudness is measured by SPL(Sound Pressure Level) and the number is usually achieved by reading the SPL of a speaker that is reproducing a 1k sine wave at 1 watt and 1 meter in distance. Sometimes SPL measurement will read as “Sensitivity” From this measurement and the ohm rating you can tell how much wattage you will need to reach a certain SPL at a certain distance. Sound pressure - Wikipedia